The present invention relates to a process for preparing copolymers, in particular to a process for preparing copolymers of ethylene with alpha-olefins having a bimodal molecular weight distribution.
Bimodal or multimodal polyolefins with broad molecular weight distributions are obtained commercially using Ziegler catalysts in slurry or gas phase polymerisation processes in which different operating conditions, are employed. Such processes are known as cascade processes. Polymers obtained in such processes often selectively incorporate comonomers in one part of the molecular weight distribution. Other parts of the polymer often contain little or no comonomer incorporation.
These polymers have been found to offer advantages in processability and tear, impact, stress crack and fracture properties depending upon the polymer application envisaged.
The cascade process relies upon different operating conditions, often using two separate reactors. It would be advantageous to be able to produce such polymers in a single reactor preferably in the gas phase under steady state conditions.
Bimodal polyolefins may be prepared by using combinations of polymerisation catalysts as components, for example a metallocene and a Ziegler catalyst or alternatively two different metallocene catalysts may be used. Such catalyst systems may be referred to as multisite catalysts. In such systems the different catalyst components must be able to produce polyolefins of different molecular weights under a single set of reactor process operating conditions, so that a bimodal molecular weight distribution is formed. Typically the low molecular weight portion of such bimodal polymers are derived from the metallocene component(s) of the catalyst.
It would also be desirable to use multisite catalysts to prepare polyolefins with specific comononer distributions across their bimodal molecular weight distributions. Unfortunately metallocene components conventionally used in multisite catalysts are known for the ability to incorporate high quantities of comonomer relative to Ziegler catalysts, resulting in bimodal MWD polymers in which comonomer is concentrated in the low MW portion.
It has now been found that certain metallocene components have a low propensity for incorporating comonomer into bimodal polymer even in the presence of high concentrations of comonomer and may be utilized to control the comonomer distribution.
The comonomer distribution is dependent upon the comonomer incorporation properties of the individual components of the multisite catalyst. Hence by using such metallocene components having a low propensity for incorporating comonomer, polymers may be obtained which typically exhibit a bimodal comonomer distribution in which the comonomer is more evenly distributed over the MWD or is even concentrated in the high molecular weight component.
Thus according to the present invention there is provided a process for preparing bimodal molecular weight distribution-copolymers of ethylene with alpha-olefins having 3 to 20 carbon atoms, said copolymers having:
(a) a comonomer distribution wherein the comonomer level at the mid-position of the low molecular weight component is  less than 3 times the level at the mid position of the high molecular weight component, and
(b) a total average comonomer content in the range 0.5-20 short chain branches (SCB)/1000 C atoms
and characterised in that said process is carried out in the presence of a supported multisite catalyst.
The xe2x80x9ccomonomer levelxe2x80x9d defined in (a) represents the comonomer content, measured in short chain brafiches per thousand backbone carbon atoms (SCB/1000 C), of the polymer at the specified molecular weight which is independent of the proportion of the total polymer represented by the polymer at that molecular weight.
The xe2x80x9ctotal average comonomer contentxe2x80x9d defined in (b) is the average comonomer content, in SCB/1000 C, of all polymer over the entire molecular weight range.
The multisite catalyst is defined as comprising two active components for example a Ziegler catalyst component producing a high molecular weight polymer component and a metallocene component producing a low molecular weight polymer component. The metallocene component may also be comprised of two or more different metallocene species provided that together they provide the required low molecular weight polymer component.
Examples of metallocenes suitable for use in the present invention are represented by the following Formulae:
(C5R5)(C5R15)MY2xe2x80x83xe2x80x83(I)
(C5R2H3)(C5R12H3)MY2xe2x80x83xe2x80x83(II)
(C5R4)Z(C5R14)MY2xe2x80x83xe2x80x83(III)
(C5RmH5xe2x88x92m)(C5R1nH5xe2x88x92n)MY2xe2x80x83xe2x80x83(IV)
wherein,
C5R5 and C5R15 etc represent a cyclopentadienyl ligand,
R and R1 alkyl, aryl, alkylaryl, alkenyl, or haloalkyl and may be the same or different,
z=bridging group comprising CX2, SiX2, GeX2 etc,
X=hydrogen or as defined by R and R1 above,
M=Zr, Ti or Hf,
Y=univalent anionic ligand for example halide, alkyl, alkoxy, etc.
and wherein in Formula (II) at least one of R and R1 has xe2x89xa73 carbon atoms and in Formula (IV) m=3 or 4.
Examples of suitable metallocenes as represented by Formula (I) and (II) are bis(pentamethylcyclopentadienyl) zirconium dichloride and bis(1-propenyl-2-methylcyclopentadienyl) zirconium dichloride respectively.
These metallocenes are represented by the Formula: 
By using the multisite catalysts of the present invention, polymer compositions containing a lower absolute comonomer incorporation level may be obtained for a given set of reaction conditions.
The metallocenes may be prepared in accordance with literature methods eg J E Bercaw et al JACS 100, 10, 3078, Canadian Journal of Chemistry 69, 1991, 661-672 and E Samuel et al J. Organometallic Chem. 1976, 113, 331-339.
Bimodal distribution is defined as relating to copolymers which show a substantially different molecular weight distribution between the low and the high molecular weight components.
Typically the low molecular weight component has a mid-position in the range 1000-300,000 preferably in the range 5000-50,000 and the high molecular weight component has a mid-position in the range 100,000-10,000,000 preferably in the range 150,000-750,000.
The total average comonomer content is preferably in the range 1 to 20 SCB/1000 C atoms.
The multisite catalyst for use in the present invention may be used in the presence of suitable co-catalysts. Suitable co-catalysts are organometallic compounds having a metal of Group IA, IIA, IIB or IIIB of the periodic table. Preferably, the metals are selected from the group including lithium, aluminium, magnesium, zinc and boron. Such co-catalysts are known for their use in polymerisation reactions, especially the polymerisation of olefins, and include organo aluminium compounds such as trialkyl, alkyl hydrido, alkyl halo, alkyl alkoxy aluminium compounds and alkyl aluminoxanes. Suitably each alkyl or alkoxy group contains 1 to 6 carbons. Examples of such compounds include trimethyl aluminium, triethyl aluminium, diethyl aluminium hydride, triisobutyl aluminium, tridecyl aluminium, tridodecyl aluminium, diethyl aluminium methoxide diethyl aluminium ethoxide, diethyl aluminium phenoxide, diethyl aluminium chloride, ethyl aluminium dichloride, methyl diethyoxy aluminium and methyl aluminoxane.
The preferred compounds are alkyl aluminoxanes, the alkyl group having 1 to 10 carbon atoms, especially methyl aluminoxane (MAO) and trialkyl aluminium compounds eg trimethylaluminium. Other suitable co-catalysts also include Bronsted or Lewis acids.
The co-catalyst may be mixed with the supported multisite catalyst. For example the metallocene component and co-catalyst (eg MAO) may be added to a supported Ziegler catalyst. During the subsequent polymerisation process a second cocatalyst (eg trimethylaluminium) may be added to the reaction medium.
Catalyst supports used with the multisite catalyst may comprise a single oxide or a combination of oxides or metal halides. They may also be physical mixtures of oxides or halides. The supports may have a high surface area (250-1000M2/g) and a low pore volume (0-1 ml/g) or a low surface area (0-250M2/g) and high pore volume (1-5 ml /g) or preferably high surface area (250-1000M2/g) and high pore volume (1-5 ml/g) (mesoporous). Preferred support materials are silica, alumina, titania, boria and anhydrous magnesium chloride or mixtures thereof, although any support used in heterogeneous catalysis/polymer catalysis may be employed.
The support may undergo a pretreatment to modify its surface eg thermal or chemical dehydroxylation or any combination of these, using agents such as hexamethyldisilazane and trimethylaluminium. Other reagents that can be used are triethyaluminium, methylaluminoxane and other aluminium containing alkyls, magnesium alkyls especially dibutyl magnesium and alkyl magnesium halides, zinc alkyls and lithium alkyls. Different impregnation regimes may be used to add the surface treatment and subsequent catalyst impregnation. Impregnation may take place sequentially or in a number of separate steps or in a single step using any method known in the prior art including vapour phase treatment/impregnation techniques.
The component of the multisite catalyst which provides the high molecular weight component may suitably be a conventional Ziegler catalyst, a Phillips catalyst or alternatively another metallocene catalyst. Preferably the high molecular weight component is a Ziegler catalyst.
A suitable catalyst is disclosed in European Application No. EP 595574.
The multisite catalyst used in the process according to the present invention may be used to produce polymers using solution polymerisation, slurry polymerisation or gas phase polymerisation techniques. Suitably alpha olefins used in the copolymerisation may be butene-1, hexene-1, 4-methyl pentene-1 octene-1 or higher xcex1-olefins which may be provided in-situ. Methods and apparatus for effecting such polymerisation reactions are well known and described in, for example, Encyclopaedia of Polymer Science and Engineering published by John Wiley and Sons, 1987, Volume 7, pages 480 to 488 and 1988, Volume 12, pages 504 to 541. The multisite catalyst composition according to the present invention may be used in similar amounts and under similar conditions to known olefin polymerisation catalysts.
The polymerisation may optionally be carried out in the presence of hydrogen. Hydrogen or other suitable chain transfer agents may be employed in the polymerisation to control the molecular weight of the produced polyolefin. The amount of hydrogen may be such that the ratio of the partial pressure of hydrogen to that of olefin(s) is from 0.0001-1, preferably 0.001-0.1.
Typically, the temperature is from 30 to 110xc2x0 C. for the slurry or xe2x80x9cparticle formxe2x80x9d process or for the gas phase process. For the solution process the temperature is typically from 100 to 250xc2x0 C. The pressure used can be selected from a relatively wide range of suitable pressure, eg from sub-atmospheric to about 350 MPa. suitably, the pressure is from atmospheric to about 6.9 MPa, or may be from 0.05-10, especially 0.14 to 5.5 MPa. In the slurry or particle form process the process is suitably performed with a liquid inert diluent such as a saturated aliphatic hydrocarbon. Suitably the hydrocarbon is a C4 to C10 hydrocarbon, eg isobutane or an aromatic hydrocarbon liquid such as benzene, toluene or xylene. The polymer is recovered directly from the gas phase process, by filtration or evaporation from the slurry process and by evaporation from the solution process.
The process according to the present invention is particularly suitable for use in the gas phase.
By using the multisite catalysts of the present invention copolymer compositions containing a lower absolute comonomer incorporation level than comparable compositions when prepared under the same polymerisation reaction conditions may be prepared. This can lead to enhanced product properties for example higher stiffness for high density tough film. The ability to operate in the presence of a relatively high comonomer concentration yet produce products containing a low absolute comonomer level is also advantageous because it allows access to a wider range of molecular weight distribution for a given density range.